Renal, transcapillary, net exchange in the dog.

The transcapillary net exchanges of inulin, para-ajiiinohippurate, thiocyanate and heavy water were determined in the renal circulation. It was found that the tracer substances larger than heavy water diffused more freely through renal than through previously studied peripheral or pulmonary capillaries. The results are consistent with the concept of a larger pore size in renal capillaries. P REVIOUS studies have been concerned with the net, transcapillary exchange of diffusible tracer substances in the human forearm , 1 ' 2 and in the lungs. 8 The patterns of net exchange have been elucidated for heavy water, 2 ' B thiocyanate, 2 ' 3 sodium 24 , 4 rubidi-um 86 , * S 3 *-labeled sulfate,* S S4-labeled methi-onine, 5 and inulin. 4 With this experience as a background it seemed of interest to explore the renal circulation using similar technics. There has long been evidence that renal cap-illaries have a greater permeability than peripheral capillaries. 6-T METHODS Mongrel dogs were used weighing 10 to 16 Kg. The animals were anesthetized with sodium pento-barbital, 25 mg./Kg. A femoral vein was cathe-terized and then the ipsilateral renal vein. The two catheters were connected so that the entire renal venous drainage on that side was diverted into the femoral vein. 8 The mixture of tracer substances was injected into the renal artery at a constant rate over a 3 to 6 sec. period. Immediately prior to the injection the coupling between the renal and femoral vein was broken and the renal venous outflow collected at 2 see. intervals into a series of heparinized, paraffined test tubes. Analyses of the injected mixture and of the blood samples and methods of calculation were carried out as described previously. 1 '' The salient features may be summarized as follows: the labeled substances under study which diffuse across capillary walls are mixed with a substance which does not pass through capillary walls to any appreciable extent in a single circulation (T-1824, I m-labeled human serum albumin (RISA) or Cr'Mabeled red cells). A portion of the mixture was saved for later analyses and the remainder injected into the renal artery. The venous samples and the aliquot of the injected mixture were analyzed for their concentrations of the various labeled substances. The "expected" concentration of each of the diffusible substances in each sample was then rt calculated as follows: C l x =-^ X C e m , where <7' …